I can't speak for Weber and I don't understand the kettle, but my opinion on why they don't include a thermometer on the WSM is because you don't need one. If you follow their directions, it's almost certain that the WSM will stay in a normal BBQ range of about 225º to 275º. Basically, if you have a good fire going and a water pan full of water, the WSM hold normal smoking temps.
It's only us minion method, sand in the pan, don't let my temp fluctuate pit masters that need a thermometer. And by not putting one on the unit at the factory, you get your choice. If drilling a hole is a concern, there are other choices that require no drilling. Part of the fun is to make it your own.
Show us a pic when you get yours mounted.